What causes mudslides?

A devastating mudslide has killed at least 17 people in Montecito, Santa Barbara, but why do mudslides happen and what made this one so dangerous?

#CAstorm– A damaged home off Olive Mill Road in Montecito is shown with receding flood waters still flowing past, as SB County Sheriffs Deputies are staged to assist with rescue/recovery efforts if needed. pic.twitter.com/TLLPDiLeRy